The present invention relates generally to electronic circuits, and, more particularly, to an electronic circuit for detecting mismatch between input signals.
Electronic circuits such as microprocessors, microcontroller units (MCUs), system-on-chips (SOCs), and application specific integrated circuits (ASICs) are widely used in applications including industrial applications, automobiles, home appliances, as well as mobile and handheld devices. When an automobile crashes, a safety or restraint system in the automobile is usually employed to protect the occupants from harm, such as deploying an airbag. Such safety mechanisms are becoming increasingly important as automobiles are required to comply with various functional safety standards including NHTSA FMVSS 208 (the Federal Motor Automobile Safety Standards issued by the National Highway Traffic Safety Administration) and ISO26262 (a Functional Safety standard by the International Standards Organization, titled “Road Automobiles—Functional safety”).
Engine control units (ECUs) are used to monitor and control critical functions of the automobiles including deployment of airbags based on multiple sensor outputs, e.g., impact detection sensors. FIG. 1 shows a conventional dual-channel impact detection system 100 used for deploying airbags. The impact detection system 100 includes first and second sensors 102a and 102b, first and second analog-to-digital converters (ADCs) 104a and 104b, first and second processors 106a and 106b, and an ECU 108. The first processor 106a includes a first sinc filter 110a, a first trimmer 112a, a first infinite impulse response (IIR) filter 114a, and a first high pass filter (HPF) 116a. The second processor 106b includes a second sinc filter 110b, a second trimmer 112b, a second IIR filter 114b, and a second HPF 116b. 
The first and second sensors 102a and 102b may be mounted at various places of the automobile, such as the front or sides, and in the ECU at the center of gravity of the automobile to sense a frontal impact, a side impact, or a vehicle roll-over. The sensors used for crash detection typically are acceleration, gyroscopic and/or pressure sensors. In the impact detection system 100, the first and second sensors 102a and 102b are acceleration sensors that generate respective first and second output signals upon detection of a crash.
The first and second ADCs 104a and 104b, which may be sigma-delta ADCs, receive the first and second output signals and generate first and second digital signals. The first and second processors 106a and 106b receive the first and second digital signals. More particularly, the first and second sinc filters 110a and 110b, which may be linear phase finite impulse response (FIR) filters or a cascaded structure of integrators and differentiators, receive the first and second digital signals, and pass low frequency components of these signals, decimate these signals, and generate first and second sinc output signals.
The first and second trimmers 112a and 112b receive the first and second sinc output signals. The first and second trimmers 112a and 112b include multipliers and adders for compensating gain, DC offset, and non-linearity errors that may be introduced by the sensors 102a and 102b and the ADCs 104a and 104b, and generate first and second trim output signals.
The first and second IIR filters 114a and 114b receive the first and second trim output signals and pass low frequency components of these signals. The first and second IIR filters 114a and 114b include multipliers, adders, and memory elements for performing the filtering operation on the trim output signals. The first and second IIR filters 114a and 114b then generate first and second filter output signals. The first and second HPFs 116a and 116b receive the first and second filter output signals and pass high frequency components of these signals and generate first and second processed output signals. The first and second processed output signals are indicative of the instantaneous change of acceleration of the automobile detected by the first and second sensors 102a and 102b. 
The ECU 108 is connected to the first and second HPFs 116a and 116b for receiving the processed output signals. The ECU 108 uses the processed output signals to run pre-stored impact detection algorithms to determine if deployment of airbags is necessary.
Generally, the first and second trimmers 112a and 112b are implemented using multipliers, adders, and intermediate registers. The first and second IIR filters 114a and 114b and the first and second HPFs 116a and 116b are implemented using memory elements, adders, multipliers, and intermediate registers. Multipliers are an expensive component and require a large circuit area. The intermediate registers used to store intermediate multiplication results also use up circuit area.
It would be advantageous to have a system for detecting a mismatch between input signals that consumes less chip area.